U.S. Pat. No. 4,246,374 discloses thermally stable polyglutarimide polymers and a practical process for their preparation. These polymers, especially those prepared from poly(methyl methacrylate) and monomethylamine or ammonia in a devolatilizing extruder, are useful as clear glazing or protective materials and have acceptable impact and modulus properties with a higher service temperature than other clear thermoplastics. U.S. Pat. No. 4,727,117 discloses that a reduction of acid content in such polyglutarimides by a post-polymerization reaction affords poly(glutarimides) of improved chemical resistance, including water resistance, miscibility, and melt flow.
Poly(glutarimides) are among the best thermoplastics in resistance to weathering. However, they exhibit surface degradation on exposure to ultraviolet (UV) radiation from the sun or from artificial light sources that causes cracking or crazing of the polymer surface, eventually affecting appearance, transparency and resistance to impact. Conventional ultra-violet stabilizers absorbing in the spectral range of the damaging ultraviolet radiation (ca. 300 nm wavelength) have proved effective in stabilizing samples not exposed to high temperatures, including stabilization of the interior of the polymer specimen.
Glutarimides are used, for example, in the protection of high intensity discharge lamps, such as those used in street lamps, automotive lighting, and the like. Such lamps generate large amounts of heat and emit a portion of their radiation in the UV spectrum. Since it is desirable to use as small an enclosure as possible, the protective unit is designed so as to operate at as high a temperature as possible consistent with the maintenance of the plastic's shape and modulus. Poly(methyl methacrylate), otherwise a good choice for stability and clarity, fails because the service temperature is too low, and polycarbonate, which has adequate service temperature, fails because it cannot be readily stabilized against UV light.
Polyglutarimides, especially those with a high proportion of (N-methyl)dimethylglutarimide mers, exhibit adequate thermal stability up to at least about 20.degree. C. below the glass temperature. It is desirable to extend their stability to ultraviolet exposure, to lower the cost and extend the timing for replacement,especially as many potential uses are in installations difficult to access, such as street lights and the like.
Stabilization of polyglutarimides against these severe conditions may be accomplished with conventional UV stabilizers, such as benzotriazoles, but amounts exceeding one weight percent are required, adding additional cost. Further, certain benzotriazole stabilizers containing an aromatic hydroxy group, after processing into polyglutarimide, have a higher initial color rating than desired.
It has been known to stabilize methyl methacrylate polymers with hindered amine stabilizers which do not absorb in the sun-tanning wave length to prepare protective covers for ultra-violet lamps in sun-tanning baths; see U.S. Pat. Nos. 4,550,136 and 4,593,064 and European Patent Application 164,663.
Japanese Kokai 58-57445 teaches a specific type of hindered amine stabilizer containing a disulfide bond as suitable for stabilization of plastics against light, although not against the specific combination of heat and light of concern to the present invention. In a list of polymers is the term "polyimide", but it is clear that the term refers to linear imide polymers containing aromatic rings joined by imide functionality, not the aliphatic imides employed in this invention.
Japanese Kokai 62-151465 teaches stabilization of polymers having an imide group on the side chain with a stabilizer combination of a hindered phenol of MW above 300 and a broad class of photostabilizers, including benzotriazoles and hindered amines. The imide groups on the side chain includes five-membered imides derived from maleic anhydride and maleimide, and refers to, but does not exemplify, imides prepared by imidization of a polymer having methacrylic acid groups, but not methyl methacrylate groups. There is no teaching of the singular effect at high use temperatures imparted by the stabilizers of the present invention, nor any teaching that the hindered amine may be used as the sole stabilizer.
Japanese Kokai 58-183729 broadly teaches the use of organophosphorus compounds to improve impact strength and lessen thermal discoloration of polymers having imide side chains. It does not describe the specific imide polymers employed here, nor exemplify any glutarimide polymer. It does not suggest the use of the hindered amines of the present invention.
It is an object of this invention to stabilize polymers against surface crazing on exposure to ultraviolet light, especially at temperatures near the glass temperature of the polymer. It is a further object to provide a stabilizer which does not contribute to color of the polymer on weathering, is stable to exposure to the weather such as rain, hail, sleet, and snow, that stabilizes against loss of polymer toughness, and which is useful at low levels. It is a further object to provide a stabilized poly(N-lower alkyl)dimethylglutarimide or poly(N-hydrogen)dimethylglutarimide polymer useful in the manufacture of protective housings for lamps.